Here's my view after owning a R34GTT for 5 years and taking it from Stock import to 500hp GTR-Engined uber beast

with a world renowned website for R34 owners (now gone sadly).
Insurance - Specialist is pretty much the only way to go I'm afraid. I was 26 when I imported it in Apr 2003 and the insurance (group 20 car) was £1000 back then. Insurance prices have driven up considerably since then and my last quote in Dec 2008 at 32 (still full NCB) was £1200.
Overall Car - Without doubt the GTT has been the most "Fun" car I've ever driven (inc GTR's) but I'm heavily biased as I used mine for Drifting [hipster]Before it became the UKD1[/hipster] and the GTR's I drove didn't like that (attesa went mental).
Engine - Solid as a rock! I started at stock then did the usual Induction/exhaust/1 Bar etc within the 1st few months. Then I decided that 400hp was the way forward so got Rod Bell to stick a GReedy turbo ii with an FMIC, Apexi PowerFC etc. I ran it like this for about 1 1/2 years before I broke it by fitting an Oil Cooler incorrectly (pipe blew off while driving). Instead of a simple rebuild (I felt 400hp on a GTT RB25 lacked enough torque) I wanted to do something unique and had a RB26DETT fitted (1st in UK) by Rising Sun Performance. I had this modded to run 450hp daily with a 500hp Overboost facility. It ran like that for about 3 years before I sold it. It never went wrong (apart from Me) but because I drove it hard it did get serviced (well Oil change and a quick look at) every 3 months regardless.
Looks - I never liked the standard GTT look, it was too "Normal" LOL. I had already got a mental pic in my head of what I wanted (GTR-ish looks but not the GTR's fat bum). I tried GTR alloys but took them off (didn't match the slimmer car). Until I got a FMIC and Top Secret bumper the cars front just looked empty. Once I'd finished (about 6 months after buying it) I never altered anything external as it was perfect for me. Oh and as the car was Black I had lots of genuine CF added which belnded it amazingly well.
Running costs -Not too bad. Mine never cost me any "repair" money until my own stupidity broke it. The money came from Modding which cost me well in excess of £20,000 over 5 years. After that it was petrol, I was using 3-4 50L tanks a month (8-15MPG - I drove it hard remember) and aren't sure I could justify that now. An oil service was £75 every 3 months and big service every year was £500-800. However, when I imported it the 1st service was about £1200 as I had every fluid, belt, plu, coil pack etc replaced which *may* have helped keep the running cost down through the rest of it's time with me.
Overall - Amazing cars if looked after. If you spend a bit of money straight after buying one on a good service then they are very reliable but know it's limits and don't exceed them so 1bar MAX on a standard turbo etc, let it cool down properly after a run (last mile of a journey no boost, low revs rather than sat still on a turbo timer).
Hope that helps - The one you're looking at loks like it was done properly (ie it's the sort of spec I had) and was done by Abbey who know their stuff so as long as it's been cared for there should be no issues (although I'd still get it looked at by Rising Sun who are closer to you than Abbey)
Here's my R34 pics/Vids etc - Got it featured in Banzai too - sorry about the root directory website.
Original 2003
Modded 2005
Banzai 2006